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Arrays

Goals

  • Make some arrays and do stuff with them

  • Retrieve data from arrays

Step 1

Type this in irb:
fruits = ["kiwi", "strawberry", "plum"]

An array is a list of things in square brackets, separated by commas.

We generally call the individual things in an array elements.

Type this in irb:
things = [5, 'tree', 19.5]
things.length

An array can contain all sorts of things, not just strings.

length is a method that tells you how many elements are in an array.

Type this in irb:
  empty_array = []
  empty_array.length

The simplest kind of array is the empty array.

Step 2

Type this in irb:
fruits[0]
fruits[1]
fruits[2]

Array elements are stored in order. You can retrieve them by using the square brackets to access them by their index.

Arrays are ordered: elements remain in the same order they started in.

Ruby starts counting at zero: the first element is fruits[0].

Type this in irb:
fruits.first
fruits.last

Ruby gives us some helpful ways to get the first and last element from an array.

Step 3

Type this in irb:
['salt'] + ['pepper']

Arrays can be added together with the plus operator.

Type this in irb:
fruits + ['mango']
fruits

The plus operator doesn't modify the existing array, it makes a new one. How could you write that last piece of code to also modify the fruits array?

Step 4

Type this in irb:
fruits = ["kiwi", "strawberry", "plum"]
fruits.push('apple')
fruits.pop()

Ruby has many methods for modifying arrays. What did these two methods do?

Explanation

Arrays are used whenever you need to work with a large group of similar items.

A short list of methods for Array:

length how long is this array (how many elements)
first get the first element of the array (same as array[0])
last get the last element of the array (same as array[-1])
push add a new element to the end of the array
pop remove (and return) the element at the end of the array

We are learning that different types of objects in ruby have different sets of methods. We can call pop on object of type Array, but we can't call it on an object of type Float.

Type this in irb:
[].pop
3.0.pop

In object oriented programming, each type of object shares a common list of methods. This list of methods is different for every type of object. When you attempt to call a method that is not available for the object's type, you get a NoMethodError and the interpreter stops running the program.

Further Reading

Ruby's documentation for Array

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